This invention relates generally to brush seals, and more particularly to determining if a brush seal is defective.
Brush seals include linear and annular brush seals. Annular brush seals have been used, or proposed for use, between a rotor and a surrounding casing (i.e., stator casing) of a rotary machine. An annular brush seal has the shape of a completely-circular or partially-circular ring. When the annular brush seal has the shape of a partially-circular ring, several such segments are circumferentially-arrayed to form a completely-circular ring. Each annular brush seal includes a plurality of flexible, metal-wire bristles attached to (e.g., welded to) a metal bristle holder.
Rotary machines include, without limitation, turbines for steam turbines and compressors and turbines for gas turbines. Steam and gas turbines are used to produce electricity for power companies, and gas turbines are used to produce thrust for jet aircraft. A steam turbine has a steam path which typically includes, in serial-flow relationship, a steam inlet, a turbine, and a steam outlet. A gas turbine has a gas path which typically includes, in serial-flow relationship, an air intake (or inlet), a compressor, a combustor, a turbine, and a gas outlet (or exhaust nozzle). Gas or steam leakage, either out of the gas or steam path or into the gas or steam path, from an area of higher pressure to an area of lower pressure, is generally undesirable. For example, gas-path leakage in the turbine or compressor area of a gas turbine, between the rotor of the turbine or compressor and the circumferentially surrounding turbine or compressor casing, will lower the efficiency of the gas turbine leading to increased fuel costs. Also, steam-path leakage in the turbine area of a steam turbine, between the rotor of the turbine and the circumferentially surrounding casing, will lower the efficiency of the steam turbine leading to increased fuel costs.
A worn-out brush seal allows unwanted leakage to occur which reduces the efficiency of the rotary machine thereby increasing fuel costs. Some annular brush seals have become worn in service well before their expected lifetimes. What is needed is a method for determining if a brush seal is defective because it is an early-wearing brush seal, such method being most useful if it can be applied before such defective brush seal is installed in a rotary machine.
In a first embodiment, the invention can be expressed as a method for determining if a brush seal is defective, wherein the brush seal has a set of bristles. The method includes empirically determining a stiffness value for each of two or more different subsets of the set of bristles. The method also includes calculating a variation in stiffness based on the stiffness values previously empirically determined. The method further includes identifying the brush seal as defective if the variation in stiffness previously calculated exceeds a predetermined stiffness-variation criteria.
In a second embodiment, the invention can be expressed as apparatus for determining if a brush seal is defective, wherein the brush seal has a set of bristles. The apparatus includes a load transducer, a displacement transducer, and a clamp for securing the brush seal. The load transducer has a contact head which is sized to contact less than twenty percent of the bristles of the set of bristles when the contact head and the free ends of the bristles are brought into contact. The displacement transducer is positioned to measure the displacement of the free ends of the bristles when the contact head and the free ends of the bristles are brought into contact. The clamp is positioned to present the free ends of a portion of the bristles of the set of bristles to the contact head of the load transducer.
Several benefits and advantages are derived from the invention. Applicants discovered that a brush seal having a large variation in bristle stiffness among different bristles is a defective brush seal which will have its more stiff bristles wear faster than its less stiff bristles. Such uneven wear develops quickly for a large variation in bristle stiffness and can render the brush seal unfit for proper sealing soon after installation. For an annular brush seal installed in an aircraft gas turbine application, a loss of a million dollars a year can result from using worn-out brush seals. Applicant""s method and apparatus determine if a brush seal is an early-wearing brush seal before it is installed so that only long-lasting brush seals are actually installed and used.